The majority of breast cancers rely on estrogen to stimulate their growth and survival. Estrogen is produced from precursor hormones by the aromatase enzyme, whose action can be blocked with aromatase inhibitors (AIs). Unfortunately ~40% of breast cancer patients are resistant to this treatment and their breast tumors either continue to grow or recur despite depletion of circulating estrogen. The precise cause of AI-resistance is not known. Our lab aims to determine the mechanisms that allow breast cancer cells to survive in estrogen- depleted conditions. We have previously reported the generation of an AI-resistant breast cancer cell line, MCF-7:5C, which was isolated under estrogen-free conditions from the estrogen-dependent cell line MCF-7. The MCF-7:5C cell line is highly aggressive and overexpresses several interferon stimulated genes including, interferon inducible transmembrane protein 1 (IFITM1). We have found that IFITM1 is overexpressed in both this AI-resistant cell model and AI-resistant breast tumors isolated from patients. IFITM1 is not normally expressed by cells and is only produced following stimulation with interferons (IFNs). Mechanistic studies have revealed that elevated IFN? drives IFITM1 expression through the IFN? receptor (IFNAR). Most notably, loss of IFITM1 expression induces the death of the AI-resistant MCF-7:5C cells, suggesting that inhibiting this pathway might hold therapeutic promise. We hypothesize that JAK/STAT signaling from IFNAR utilizes specific interferon sensitive response elements (ISRE) in the IFITM1 promoter to drive expression and that IFITM1 overexpression allows the AI-resistant MCF-7:5C cells to maintain their aggressive phenotype in vivo. In order to address this hypothesis we propose the following specific aims: Aim 1: Identify the mechanism by which JAK/STAT signaling stimulates IFITM1 overexpression in AI-resistant breast cancer cells. Aim 2: Determine whether IFITM1 overexpression provides a functional advantage to AI-resistant cells in vivo. In aim 1 we will utilize chromatin immunoprecipitation (CHIP) assays and luciferase reporters to determine which transcription factors drive IFITM1 overexpression and to investigate the motifs utilized in the IFITM1 promoter. In aim 2, we will utilize inducible lentivirial expression vectors to produce gain and loss of function clones of our cell lines. The impact of altered IFITM1 expression on the invasive and tumorigenic potential of AI-sensitive and AI- resistant breast cancer cells will be determined using the mouse intraductal (MIND) model of invasive breast cancer progression. The insights gained from this study will further our understanding of the role that IFN signaling and IFN stimulated genes play in breast cancer prognosis and resistance to therapy. The proposed project will also provide a foundation for my future as a Physician Scientist in the field of Oncology.